1. Field of the Invention
The present invention relates to a hydraulic power steering apparatus.
2. Description of the Related Art
Hydraulic power steering apparatuses have been proposed for assisting steering operation of vehicles and the like. In general, the conventional hydraulic power steering apparatus mainly consists of a pump 50, a reservoir 90, a rotary control valve 10 and a power cylinder 30, as shown in FIG. 1. The pump 50 is for supplying operating fluid for producing assist power in the steering operation of a driver. The reservoir 90 is for storage of the operating fluid. The rotary control valve 10 locates between the pump 50 and the reservoir 90, and controls the operating fluid in accordance with the steering operation. The power cylinder 30 works in response to the operating fluid controlled by the rotary control valve 10.
The rotary control valve 10 adopts a center-closed-type for giving the driver steady steering feeling in the vicinity of the neutral position of the steering wheel (refer to FIG. 2). The center-closed-type of the rotary control valve 10 substantially closes an inlet port 10a leading the operating fluid from the pump 50 in the vicinity of the neutral position of the steering wheel to prevent the operating fluid for the power cylinder 30. Since difference pressure in the power cylinder 30 is maintained to be extremely low or substantially zero in the vicinity of the neutral position, the steering feeling becomes similar to that of a non-assisted steering apparatus. Therefore, the power steering apparatus is able to give the driver steady steering feeling in the vicinity of the neutral position of the steering wheel, i.e., rigidity of the neutral position is enhanced.
However, the inventors found that the power steering apparatus with the center-closed-type rotary control valve 10 is inferior to that with a center-open-type rotary control valve 10' shown in FIG. 3 in self-return of the steering wheel to the neutral position thereof. The self-return of the steering wheel is a restoring rotation of the steering wheel caused by reaction force from wheels, i.e., self-aligning-torque, so as that the steering wheel automatically returns to the neutral position after the driver stops turning the steering wheel.
The center-open-type rotary control valve 10' shown in FIG. 3 opens an inlet port 10a', cylinder ports 10b' and return ports 10c' in the neutral position thereof. This configuration allows two discharging passages A' and B' in each of the cylinder port 10b' to release the operating fluid under the self-return of the steering wheel. When the steering wheel returns to the neutral position, i.e., the self-return starts, a displaced power piston 330' returns to the neutral position of the power cylinder 30'. For example, the power piston 330' shown by broken lines moves to the neutral position shown by solid lines. The moving power piston 330' discharges the operating fluid from a left-side cylinder chamber to the left-side cylinder port 10b' (refer to a broken arrow line of FIG. 3). Under this state, the operating fluid is discharged from the passages A' and B' in the cylinder port 10b'.
On the other hand, the center-closed-type rotary control valve 10 shown in FIG. 2 closes the inlet port 10a in the neutral position thereof. This configuration allows only one discharging passage A in each of a cylinder port 10b to release the operating fluid under the self-return of the steering wheel. When the steering wheel returns to the neutral position, a displaced power piston 330 returns to the neutral position of the power cylinder 30. For example, the power piston 330 shown by broken lines moves to the neutral position shown by solid lines so as to discharge the operating fluid from a left-side cylinder chamber to the left-side cylinder port 10b (refer to a broken arrow line of FIG. 2). Under this state, the operating fluid is discharged from the passage A in the cylinder port 10b.
Therefore, the center-closed-type rotary control valve 10 substantially has a half of area for discharging the operating fluid compared with the center-open-type rotary control valve 10'. This structural difference gives the center-closed-type rotary control valve 10 the sluggish self-return of the steering wheel.